1. Technical Field
The present technical field relates to a surface profile measuring apparatus and a surface profile measuring method using a phase-shifting method.
2. Background Art
A schematic diagram illustrating surface profile measurements using a phase-shifting method according to the related art is shown in FIG. 12.
A beam from a light source 100 shown in FIG. 12 is split by a half mirror 101 and the split beams are applied to a sample surface 104 and a reference surface 102, respectively. Return beams from the sample surface 104 and the reference surface. 102 are superimposed through the half mirror 101. When the two return beams are superimposed, optical interference occurs. An interference intensity signal of the optical interference is detected by the use of a CCD 105. The interference intensity signal varies in a sinusoidal shape depending on the difference in optical path length between the reference surface 102 side and the sample surface 104 side. The difference in optical path length corresponding to one wavelength constitutes a sine wave of a cycle. In the phase-shifting method, the height of the sample surface 104 is measured by detecting the initial phase of the sine wave.
The initial phase of a sine wave is detected as follows. The reference surface 102 is shifted in x axis direction by a piezoelectric stage 103. The optical path length is changed with this shift and the resultant variation in the interference intensity signal is detected. The initial phase is detected from this variation. Specifically, the reference surface 102 is shifted in four steps by every ⅛ wavelength of the beam from the light source 100. When the interference intensity signals detected at this time are defined as I1, I2, I3, and I4, the initial phase φ is calculated as tan φ=(I1−I3)/(I2−I4).
The difference in optical path length s between the return beams from the sample surface 104 and the reference surface 102 is calculated as s=φ/2π×λ from the initial phase φ. Here, since the phase is repeated in the range of 0 to 2π (rad), the measureable height is a ½ wavelength and can be measured only in a very narrow range in the height direction. In order to broaden the measurement range, for example, a multi-wavelength phase-shifting method of converting the wavelength into plural wavelengths and carrying out a phase-shifting method at each wavelength is known (see U.S. Pat. No. 4,832,489).
However, in the related art, it is necessary to change the scanning dose to the reference surface 102 depending on the wavelength. Accordingly, it is necessary to repeat the scanning operations with different scanning doses depending on the wavelengths. Therefore, there is a problem in that the measurement time increases when many types of wavelengths are employed to obtain a broad measurement range in the height direction.